In recent years, a variety of power storage devices, for example, storage batteries such as lithium-ion storage batteries, lithium-ion capacitors, and air cells have been actively developed. In particular, demand for lithium-ion storage batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for electronic devices, for example, portable information terminals such as mobile phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion storage batteries are essential as rechargeable energy supply sources for today's information society.
However, general lithium-ion storage batteries each have high energy density and each include an organic solvent which may catch fire at high temperatures as an electrolyte solution; thus, the lithium-ion storage batteries may generate heat, catch fire, or explode if a protection circuit which controls charge and discharge causes malfunction or a cell is damaged, for example. Such accidents are often reported.
An all-solid-state battery including not an electrolyte solution but a solid electrolyte has been researched as a battery that is less likely to cause such accidents. For example, a storage battery in which a polymer electrolyte having lithium ion conductivity is used as a solid electrolyte has been researched.
However, even in a lithium-ion storage battery including a polymer electrolyte, the battery characteristics might be significantly degraded when the temperature of the battery becomes low and the ionic conductivity is significantly decreased. For example, although polyethylene oxide (PEO) is known as a polymer that can be used for a lithium-ion storage battery (see Patent Documents 1 to 3), the lithium-ion storage battery used at low temperatures may cause a problem of a decrease in the ionic conductivity. The melting point of PEO is approximately 60° C., and PEO is dangerous when melted because a short circuit might be caused between electrodes; thus, PEO can be used only in a narrow temperature range in the first place.